This specification relates to polishing pad windows useful for monitoring polishing rate and detecting polishing endpoints. In particular, it relates to a window configuration useful for limiting polishing defects or useful for reducing variation in signal transmission.
Polyurethane polishing pads are the primary pad-type for a variety of demanding precision polishing applications. For example, polyurethane polishing pads have high strength for resisting tearing; abrasion resistance for avoiding wear problems during polishing; and stability for resisting attack by strong acidic and strong caustic polishing solutions. These polyurethane polishing pads are effective for polishing multiple substrates, including the following: silicon wafers, gallium-arsenide and other Group III-V semiconductor wafers, SiC, patterned wafers, flat panel displays, glass, such as sapphire and magnetic storage disks. In particular, polyurethane polishing pads provide the mechanical integrity and chemical resistance for most polishing operations used to fabricate integrated circuits. Unfortunately, these polyurethane polishing pads tend to lack sufficient transparency sufficient for laser or optical endpoint detection during polishing.
Since the mid 1990s, optical monitoring systems with endpoint detection have served to determine polishing time with laser or optical endpointing for semiconductor applications. These optical monitoring systems provide in-situ endpoint detection of a wafer substrate during polishing with a light source and a light detector. The light source directs a light beam, passing it through a transparent window toward the substrate being polished. The light detector measures light reflected from the wafer substrate that passes one more time back through the transparent window. An optical path is formed from the light source, through the transparent window, onto the substrate being polished, the reflected light passing through the transparent window again and into the light detector.
Typically, the transparent window is coplanar with the polishing surface of the polishing pads. Alternative designs, however contain a recess between the window and the wafer substrate. During polishing, this recess fills with slurry. If the recess is too deep, then the slurry, together with polishing debris, can block or diffuse the optical path and there can be insufficient signal strength to achieve reliable endpoint detection. The accumulated polishing debris on a recessed window surface can scratch the wafer substrate and create defects in the resulting semiconductor.
There remains a need for a window having improved optical signal strength with a decreased risk of creating polishing defects in the wafer.